1. Photosynthesis
• Plants and Oxygen
• Plant Respiration
• Parts of Photosynthesis
• Light Reactions
Photosynthesis
Photosynthesis is essential to all life on earth; both plants and
animals depend on it. It is the only biological process that can
capture energy that originates in outer space (sunlight) and
convert it into chemical compounds (carbohydrates) that every
organism uses to power its metabolism.
Photosynthesis
Photosynthesis uses
carbon dioxide and water
to assemble carbohydrate
molecules and release
oxygen as a waste
product into the
atmosphere.
The Plant Cell
Nucleus
Central Vacuole
Cell Wall
Chloroplasts
Plants and Oxygen Production
•Cyanobacteria, like these in
Yellowstone National Park, were
the world’s first Oxygen Producers.
•This Oxygen Revolution transformed early earth’s
atmosphere.
•It was good for some (allowing for increased efficiency
with O2 respiration) but bad for others (in the form of
mass extinction of anaerobes)
Stromatolites
Stromatolites are special
rock-like structures that form
in shallow water.
• They are formed by cyanobacteria that use water,
carbon dioxide, and sunlight to create their food, and
put out oxygen as a by-product.
• The Earliest Evidence of Stromatolites: 3.5 BYA!
This world map shows Earth’s distribution of photosynthesis as seen via
chlorophyll a concentrations. On land, this is evident via terrestrial plants,
and in oceanic zones, via phytoplankton.
Percentage of Earth's Surface Area
Upwelling zones
Algal beds and reefs
Estuaryu
Lake and stream
Swamp and marsh
Temperate evergreen forest
Temperate deciduous forest
Tropical seasonal forest
Tundra
Woodland and shrubland
Temperate grassland
Boreal forest
Cultivated land
Savanna
Pretty Big!
Tropical Rainforest
Extreme Desert, Rock, Sand, Ice
HUGE!
Continental Shelf
Open Ocean
0
10
20
30
40
50
60
70
Average net primary production (g/m2/yr)
Upwelling zones
Algal beds and reefs
Estuaryu
Lake and stream
Swamp and marsh
Temperate evergreen forest
Temperate deciduous forest
Tropical seasonal forest
Tundra
Woodland and shrubland
Temperate grassland
Boreal forest
Cultivated land
Savanna
Tropical Rainforest
Extreme Desert, Rock, Sand, Ice
Continental Shelf
Open Ocean
High
Productivity!
Low Productivity!
0
500
1000
1500
2000
2500
3000
Percentage of Earth's net primary production
Upwelling zones
Algal beds and reefs
Oceans and Rainforest
are vital to oxygen
production on Earth!
Estuaryu
Lake and stream
Swamp and marsh
Temperate evergreen forest
Temperate deciduous forest
Tropical seasonal forest
What will happen if
they aren’t protected?
Tundra
Woodland and shrubland
Temperate grassland
Boreal forest
Cultivated land
Savanna
Tropical Rainforest
Extreme Desert, Rock, Sand, Ice
Way higher than the others!
Continental Shelf
Open Ocean
0
5
10
15
20
25
30
Photosynthesis/Respiration
Photosynthesis:
• Plants use H2O and
carbon dioxide and
produce starch and
oxygen
H2O + CO2 =
Starch/sugar + O2
Respiration
• Animals use
starch/sugar and
oxygen, and produce
H2O and carbon dioxide
Starch/sugar + O2 =
H2O + CO2
Photosynthesis is a multi-step process
that requires sunlight, carbon dioxide
(low in energy) and water as
substrates.
Photosynthesis releases oxygen and
produces simple carbohydrate
molecules (which are high in energy)
that can subsequently be converted
dozens of other sugar molecules.
These sugar molecules contain energy
and the energized carbon that all
living things need to survive.
Stomata on the underside of a leaf
Stomata on the underside of a leaf
An open (left) and closed (right) stoma of a spider plant
(Chlorophytum colosum) leaf. When guard cells are turgid, the
stoma is open (left).
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The Chloroplast
•Most of the living world
depends on chloroplasts for its
energy!
•Two membranes on outside
•Complex membrane structure
on inside
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Photosynthesis Summary
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Photosynthesis Summary
Photosynthesis takes place in two sequential stages
In the light-dependent reactions, energy from sunlight is
absorbed by chlorophyll and that energy is converted into
stored chemical energy.
In the light-independent
reactions, the chemical
energy harvested during
the light-dependent
reactions drive the
assembly of sugar
molecules from carbon
dioxide.
Absorbed and
Reflected Light
Reflected Light
Absorbed Light
Transmitted Light
Plants are green
because
chlorophyll reflects
green light.
Excited electrons are the key to photosynthesis.
Grannum (stack of thylakoids)
Within the membrane
of each thylakoid are
countless clusters of
pigments. These
pigments are inside
Photosystems.
Photosystem
The pigments act as
antenna, bouncing
photons towards the Primary Electron Receptor
A photosystem consists of a light-harvesting complex and a reaction center.
The first photosystem of photosynthesis is called photosystem 2.
Photosystem 2
Reaction Center
Light harvesting complex
Pigments in the light-harvesting complex pass light energy (in the form of
photons) to two special chlorophyll a molecules in the reaction center.
Chlorophyll a molecules
The light excites an electron from the chlorophyll a pair, which passes to the
primary electron acceptor.
Primary Electron
Acceptor
e-
Photosystem 2
The chlorophyll molecule must get a new electron from somewhere! It’s
electron is replaced by the splitting of a water molecule. When a molecule of
water is split energy (and oxygen) is released.
For every two water molecules that are split, one
molecule of O2, the oxygen we breathe, is produced.
H
H
O
e
H
H
Meanwhile, the excited electron that was raised to the primary electron
acceptor is transferred to a mobile carrier protein, that moves it along the
electron transfer chain.
As it moves along the electron transfer chain it releases ‘works’ to produce ATP,
the currency of energy, that a cell uses. However, as it produces ATP becomes
less and less excited.
e-
ATP
Photosystem 2
ATP
ATP
At the end of the Electron Transfer Chain, the electron is no longer excited. It
enter Photosystem 1 and is excited again by the photons boosting it back into
it’s high energy state.
e-
ATP
Photosystem 2
ATP
ATP
Photosystem 1
The re-excited electron is again transferred to a mobile carrier protein, that
moves it along the electron transfer chain, however this time it is combined
with another electron, one proton, and a molecule of NADP+ to create a
molecule of NADPH.
e-
NADP+
H
e-
ATP
ATP
ATP
Photosystem 1
NADPH
Active Transport: Proton pumps
H
H
Outside of Thylakoid
Inside of Thylakoid
ATP
H
ATP
H
H
H
The ATP released during the ETC transfer is used to drive proton pumps
which sets up a concentration gradient of high H+ inside the thylakoid
(lumen) and a low H+ outside the thylakoid (stroma)
ATPase Activity
H
ADP
P
ATP
Outside of Thylakoid
Inside of Thylakoid
H
H
H
H
H
H
H
Protons (H+) flow down concentration gradient through
ATPase, an enzyme that synthesizes ATP.
Light Reactions
• Light boosts electrons in Photosystem II, high energy
electrons passed along chain of carriers
• Electrons replaced by splitting water
• Passage of electrons down chain releases energy
used to fuel proton pumps to generate ATP
• Chain ends in Photosystem I, electron energy
boosted again, passed on to NADPH
• ATP, NADPH (fuel) produced by light reactions
provide energy to power Calvin Cycle (making sugar)
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